A pneumatic tire for heavy load generally has a belt radially outwardly arranged on a carcass to reinforce a tread portion. The belt used is usually a crossing belt having multiple layers of which cords alternatively crossed with each other between the layers, or an inclined belt having a single belt layer of which cords are inclined to the tire's circumferential direction.
When such a tire is subjected to inflation pressure, a hoop effect of the belt becomes smaller at a tread shoulder portion than at a tread center portion and thus the amount of a radial growth of the belt at the tread shoulder portion becomes larger than that at the tread center portion. Consequently, the tread shoulder portion of the belt extensionally deforms relatively large in the circumferential direction, which causes a large circumferential strain on the tread rubber. As a result, there is a problem that a separation is apt to occur between the belt and the tread rubber.
Further, pneumatic tires for heavy load recently tend to have lower profiles in response to increasing demands for lowering a vehicle floor and making a drive shaft or a trailer shaft to be single wheeled instead of traditional dual wheeled. When such a low-profile tire, especially a tire having an aspect ratio of 70% or less is subjected to inflation pressure, the amount of the radial growth tends to further increase at the shoulder portion.
JP 2-208101A describes that a reinforcement layer containing meandering cords extending along the circumference direction in a wavy or zigzag shape is provided radially outwardly or inwardly on the belt or between the belt layers as a means for suppressing the radial growth of the belt at the tread shoulder portion, and it has been used up to now. This can suppress the radial growth of the belt at the tread shoulder portion, thereby improving the durability of the tread shoulder portion.
For the tires having such a configuration, it has been considered that the transversal stiffness is not necessarily reinforced during the inner pressure being applied, since a load of the circumferential tensile force is large while a load of the transversal tensile force is small during the inner pressure being applied. However, if an input such as a projection is penetrated, the tire is easily bent and pulled in the transversal direction due to its small transversal tensile stiffness, so that carcass cords, which are transversal members, are subjected to larger input forces. For example, when the tire receives a projection input caused by passing over a stone during running on the road, the tire tends to deform more in the transversal direction than in the circumferential direction as compared with the ordinal tire.
The term “projection input” herein refers to a radially inward force acting on the tread surface when a pneumatic tire runs on the road and passes over, for example, a stone.
When the tire having such a tendency runs while repeatedly incurring the projection input, the carcass which mainly contributes the transversal stiffness breaks prior to the belt which mainly contributes the circumferential stiffness. A problem is that the tire gets blowout and/or tread burst much easier in the case where the carcass breaks first than in the case where the belt breaks first.
Further, a demand for improving transport efficiency in the modern market needs enhancements of speed and load capacity of the vehicle. From this viewpoint also, the durability of the carcass may become problematic.
In this connection, it is recently noticed that, in order to improve the durability of the carcass against the projection input, the transversal stiffness of the tread portion may be enhanced to suppress the transversal deformation occurring at the time of receiving the projection input, thereby decreasing the input force per one cord in the carcass.
JP 2002-514538A, for example, describes a tire in which an additional ply having substantially radially arranged reinforcement elements of non-extensible metal is provided between belt layers. The tire, however, is provided with the transversal reinforcement layer between the belt layers, so that the effect of reinforcing the carcass is not sufficient. JP 4-356203A describes a tire comprising belt layers among which the inner most belt layer in the tire's radial direction is dividedly arranged with spacing the central region to form a split configuration, and a reinforcement layer in which radial cords are embedded, the reinforcement layer being arranged inside these belt layers in the tire's radial direction and along the carcass. The tire, however, has no reinforcement around the tire's equatorial plane at which largest deformation is observed at the time of receiving the projection input, so its durability is not sufficient. JP 2002-192910A describes a tire in which transversal reinforcement layers having cords extending at 50 to 90 degrees with respect to the tire's circumferential direction are arranged between a carcass and a belt and between the shoulder portion and the tire's equatorial plane. The object of the tire, however, is not an improvement of the durability, but an improvement of the drivability. In addition, the tire has no reinforcement around the tire's equatorial plane at which largest deformation is observed at the time of receiving the projection input, so its durability is not sufficient. Furthermore, in the tire for heavy load having such a belt configuration, the above-mentioned projection input is easily transmitted to the carcass and thus the ply cords are readily broken, which is problematic.
As a measure for enhancing a transversal stiffness of a tread portion, enlarging the diameter of carcass ply cords and increasing the number of carcass ply cords embedded in the carcass may be recited by way of example. However, these measures involve a demerit of increasing the weight of the whole tire, and a stepwise transition of the stiffness occurring at a rolled-up end of the carcass becomes greater, which involves another demerit of decreasing the durability at the rolled-up end of the carcass.